Fuel cell technology is a relatively recent development in the automotive industry. It has been found that fuel cell power plants are capable of achieving efficiencies as high as 55%. Furthermore, fuel cell power plants emit only heat and water as by-products.
Fuel cells include three components: a cathode, an anode and an electrolyte which is sandwiched between the cathode and the anode and passes only protons. Each electrode is coated on one side by a catalyst. In operation, the catalyst on the anode splits hydrogen into electrons and protons. The electrons are distributed as electric current from the anode, through a drive motor and then to the cathode, whereas the protons migrate from the anode, through the electrolyte to the cathode. The catalyst on the cathode combines the protons with electrons returning from the drive motor and oxygen from the air to form water. Individual fuel cells can be stacked together in series to generate increasingly larger quantities of electricity.
In a Polymer-Electrolyte-Membrane (PEM) fuel cell, a polymer electrode membrane serves as the electrolyte between a cathode and an anode. The polymer electrode membrane currently being used in fuel cell applications requires a certain level of humidity to facilitate conductivity of the membrane. Therefore, maintaining the proper level of humidity in the membrane, through humidity/water management, is very important for the proper functioning of the fuel cell. Irreversible damage to the fuel cell will occur if the membrane dries out.
In the PEM fuel cell, multiple fuel cells are frequently stacked in series to form a fuel cell stack. In the fuel cell stack, one side of a flow field plate serves as the anode for one fuel cell while the opposite side of the flow field plate serves as the cathode for an adjacent fuel cell. Because each flow field plate serves as both an anode and a cathode, the flow field plate is also known as a bipolar plate. Monopolar plates, such as anode coolant flow field plates, may be provided in the fuel cell stack. One side of the anode coolant flow field plate serves as an anode flow field plate. The opposite side of the anode coolant flow field plate serves as a cathode coolant flow field plate. Coolant channels of the anode coolant flow field plate and of the cathode coolant flow field plate may be combined to form collective coolant channels for cooling the fuel cell stack.
Bipolar plates for PEM fuel cells must be electrochemically stable, electrically conductive and inexpensive. The corrosion of metallic bipolar plates in the fuel cell environment accelerates the corrosion process through degradation of the membrane. The degradation products of the membrane include hydrogen fluoride (HF), which accelerates the corrosion process, causing the corrosion process to become autocatalytic in nature. 316L and other lower grades of stainless steels have been used as inexpensive bipolar plate materials.
While 316L stainless steel exhibits a fair corrosion resistance to fluoride ions, the corrosion rate increases with the increase in the fluoride ion leach out rate. This problem can be mitigated somewhat by removing the hydrogen fluoride from the fuel cell environment or by using higher grades of stainless steel which are more resistant to corrosion by fluoride ions than 316L stainless steel. The use of higher grades of stainless steel for the bipolar plate tends to increase the cost of the bipolar plate.
Various methods are known for increasing the corrosion resistance of a corrosion-susceptible substrate. For example, U.S. 20030228512 A1 discloses a method of improving the contact resistance of the surface of a stainless steel substrate while maintaining optimum corrosion resistance of the substrate by depositing a gold coating on the substrate. U.S. 20040091768 A1 discloses a method of increasing the corrosion resistance of a substrate by providing a polymeric conductive coating on the substrate. U.S. Pat. No. 6,372,376 B1 discloses a method of increasing the corrosion resistance of a substrate by providing an electrically-conductive, corrosion-resistant polymer containing a plurality of electrically conductive, corrosion-resistant filler particles on the substrate.
It has been found that coating the surface of a lower grade stainless steel bipolar plate, such as a 316L or 304L stainless steel bipolar plate, for example, with a thin layer of high-grade stainless steel or alloy imparts a high degree of fluoride ion corrosion resistance to the bipolar plate while maintaining the cost of the bipolar plate within acceptable levels.